Receiver and receiving method simplifying the interference cancellation of multi-user detection in a direct sequence code division multiple access (DS-CDMA) telecommunication system

ABSTRACT

The invention relates to a receiving method and a receiver. In the method, in order to remove interference of a received composite signal, narrowband interfering signal estimates corresponding to the interfering signals are subtracted from a narrowband composite signal. The narrowband interfering signal estimates are generated in the interference cancellation primitives of the receiver by multiplying a detected bit of the interfering signal by a cross correlation and a channel estimate of spreading codes.

This application is the national phase of international applicationPCT/F197/00634 filed Oct. 17, 1997 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to a receiving method used in a CDMA radio systemcomprising, as a transmitter and a receiver, at least one subscriberterminal and a base station, which receive composite signal that hasbeen multiplied by a spreading code into wideband, the composite signalarriving at the receiver over several paths from a plurality oftransmitters, the composite signal then representing both interferingsignals and desired signals and which composite signal is converted intonarrowband in the receiver and detected.

The invention further relates to a receiver arranged to be used in aCDMA radio system comprising at least one subscriber terminal and a basestation which are arranged to receive wideband composite signalcomprising signals propagated over several paths from a plurality oftransmitters, and which composite signal comprises both interferingsignals and desired signals and which composite signal the receiver isarranged to convert into a narrowband signal and to detect.

BACKGROUND OF THE INVENTION

In a DS-CDMA (Direct Sequence Code Division Multiple Access) the user'snarrowband data signal is modulated by a spreading code, which is morewideband than the data signal, to a relatively wide band. In systemsused, bandwidths from 1.25 MHz to 50 MHz have been employed. A spreadingcode is conventionally formed of a long pseudo-random bit sequence. Thebit rate of the spreading code is much higher than that of the datasignal and in order to distinguish spreading code bits from data bitsand symbols, they are called chips. Each user data symbol is multipliedby the spreading code chips. Then the narrowband data signal spreads tothe frequency band used by the spreading code. Each user has his/her ownspreading code. Several users transmit simultaneously on the samefrequency band and the data signals are distinguished from one anotherin the receivers on the basis of a pseudo-random spreading code.However, the signals of different users interfere with each other in thereceiver as the spreading codes are not entirely orthogonal particularlyon account of a phase shift caused by a propagation delay.

Multiple access interference of the CDMA systems are reduced, forexample, by using Multi-User Detection (MUD). There are several suchmethods and using them interference from the users cell area can best bereduced, and thus improve the capacity of the system. Known MUDsolutions are based on two main types: regeneration of a wideband signalafter preliminary detection or decorrelation in which a reverse matrixof a cross correlation matrix of the spreading codes is generated. Inthe former, interference is removed to the effect that the strongestregenerated signals are subtracted from the received wideband compositesignal. Regenerating the wideband signal, however, requires a lot ofcalculation capacity from the receiver. In the latter, interference isreduced by multiplying a received signal vector by the reverse matrix ofthe cross correlation matrix of the spreading codes, the generation ofwhich becomes more difficult as the number of users and paths increases.

SUMMARY OF THE INVENTION

An object of the present invention is to implement a solution in whichit is avoided to generate a wideband signal and to reverse a crosscorrelation matrix of spreading codes and thus simplifying theinterference cancellation of multi-user detection.

This is achieved with the method of the type set forth in the preamblecharacterized by subtracting a narrowband interfering signal estimate ofone path of at least one transmitter from a narrowband composite signalpropagated over several paths in the receiver in order to generate aninterference cancelled signal.

A receiver of the invention is characterized by comprising aninterference cancellation means comprising a plurality of interferencecancellation primitives which are arranged to generate at least onenarrowband interfering estimate signal describing interference, theinterference cancellation means being arranged to subtract a narrowbandinterfering estimate signal from the narrowband composite signal.

Great advantages are achieved with the method of the invention.Multi-user detection and interference cancellation can be performedusing simple operations without regenerating a wideband signal orreversing a cross correlation matrix. In addition, the reduction of themutual interference of the signals simplifies power control and reducestransmission power. This, in turn, reduces interference and allows alarger capacity of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail withreference to examples in the accompanying drawings, in which

FIG. 1 is a block diagram illustrating a receiver,

FIG. 2 shows an interference cancellation primitive,

FIG. 3 shows an interference cancellation primitive,

FIG. 4 shows an interference cancellation primitive,

FIG. 5 shows a phase deviation of an interfering signal and a desiredsignal,

FIG. 6 shows an adaptive interference cancellation,

FIG. 7 shows a parallel interference cancellation,

FIG. 8 shows a serial interference cancellation iteration, and

FIG. 9 shows a radio system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solution of the invention can be used particularly in a CDMA systemusing direct spreading without restricting thereto. A received compositesignal comprises signals of a plurality of users propagated over severalpaths, the signals having phase deviations regarding each other due todifferent propagation time. The signal of one path is madeinterference-free by removing the effects of the signals of other paths.When the signals of all paths have thus been cleaned free frominterference, the signals of the paths can be combined user-specificallyand strong interference-free desired signals can be obtained.

Let us now examine in more detail the theoretical basis of the solutionof the invention. A received asynchronous CDMA signal r (t) is generallyin mode

r(t)=s(t, B)+n(t),  (1)

where $\begin{matrix}{{{s\quad (t)} = {\sum\limits_{k = 1}^{K{(t)}}\quad {\sum\limits_{i = {- P}}^{P{(k)}}\quad {\sum\limits_{l = 1}^{L{({t,k})}}\quad {h_{k,l}\sqrt{\frac{E_{b}}{T_{b}}}b_{k,i}{a_{k,l}\left( {t - \tau_{k,l} - {iT}_{b}} \right)}}}}}},} & (2)\end{matrix}$

where s_(k)(t) is a multi-path propagated multi-user signal, n(t) isnoise, a_(k,I) is a user-specific spreading code wave form, B is amatrix of user bits b_(k,I)ε{−1. +1}, τ_(k,I) is delay caused byasynchrony, h_(k,I) comprises information on attenuation, E_(b) is bitenergy and T_(b) is a bit time slot. Instead of a bit, b_(k,i) canrepresent a combination of bits, or a symbol. The number of users K (t)is a function changing with time, P (k) is the number of bits to betransmitted and L (t, k) is the number of received signal componentsdepending on time and the user, i.e., L (t, k) corresponds to the pathson which the signals have propagated. L (t, k) changes as a function oftime since the number of multi-path propagated signal components ofdifferent users changes with time. Each signal can be considered as longas a spreading code period at a time.

In order to perform the interference cancellation of the inventioninformation is needed on the cross correlation between the spreadingcodes of different signals. Here the cross correlation of the spreadingcodes means also covariance or another corresponding operation measuringsimilarity or dissimilarity. The cross correlation is calculated, forexample, as follows:R_(l, l^(′), i)(k, k^(′)) = ∫_(−∞)^(+∞)a_(k)(t − τ_(k, l))  a_(k) ⋅ (t + iT_(b) − τ_(k^(′), l^(′)))  t,

where R_(I,I′,i) (k, k′) determines the correlation between user k anduser k′ in relation to these subscribers' paths I and I′, in which k isa desired user and k′ is a user causing interference and I is a desiredpath and I′ is an interference path. As the signals are limited in timeand the propagation delays are determined, whichever cross correlationsR_(I,I′,i−i′) (k, k′) obtain the value 0, when |i−i′|>2. This simplifiesboth correlation and interference cancellation, as the forming of themover the interference bit indexes i′ can be limited in the range, [i−2,i+2] comprising five bits. The multi-user receiver is described ingreater detail in the publication : U. Fawer, B. Aazhang, A MultiuserReceiver for Code Division Multiple Access Communications over MultipathChannels, IEEE Transactions on Communications, vol. 43 (2,3,4),Feb/Mar/Apr 1995, which is incorporated herein by reference.

Let us examine the interference cancellation method of the invention inmore detail. In this solution the composite signal is preliminarydetected in such a manner that the bit decisions of different paths ofdifferent users are made. The preliminary bit decisions are made, forexample, in a RAKE branch or in a matched filter. The signalinterference cancellation of one path is performed in such a manner thatthe effects of other paths are reduced from the signal, and only acleaned signal of one path remains. This operation is performed for asignal of all paths in a serial mode or in parallel, whereby the signalsof all paths being processed are cleaned from interference.

In the solution of the invention the interfering signal is subtractedfrom the composite signal when both signals are of narrowband. Thismeans that the spreading coding of the composite signal is decoded, butthe signal is, however, preferably modulated by Binary Phase ShiftKeying (BPSK) or Quadrature Phase Shift Keying (QPSK). The symbols ofthe desired signal and the interfering signal arrive at the receiverprobably as phase shifted, whereby more than one symbol of theinterfering signal interferes with a symbol of one desired signal. Thenthe effect of the symbols of the interfering signal, by theirinterfering parts, is subtracted from one symbol of the desired signal.

Let us now examine the solution of the invention in greater detail withreference to FIGS. 1-9. FIG. 1 shows one possible block diagram of thereceiver. The receiver comprises a delay estimation means 1, acorrelation means 2, a despreading means 3, a first combining bitdecision means 4, an interference cancellation means 5, a secondcombining and hard-decision means 6, a channel decoder 7 and a channelestimation means 8. The delay estimation means 1 generates from awideband composite signal 9 a propagation delay for the signals ofdifferent paths of different users. The correlation means 2 calculatesauto and cross correlations for the spreading codes of different usersutilizing the delay information of the delay estimation means 1. In thedespreading means 3 the wideband spreading coding of the compositesignal is decoded and path-specific narrowband signals 15 are generated,which signals are demodulated in accordance with prior art and combinedinto user-specific bit decisions in the first combining and bit decisionmeans 4. In the interference cancellation means 5 interference isremoved path-specifically in accordance with the method of theinvention, after which the signals from which interference is removedare combined as transmitter, i.e., user-specific signals 38-40 and aredemodulated as preliminary bit decisions in the second combining and bitdecision means 6. The cleaned bits of a particular user can be re-fedinto the interference cancellation means 5 for a new interferencecancellation. The channel coding of interference cancelled bits isdecoded in the channel decoder 7, the output of which the bits of eachuser are. A complex channel estimate needed in interference cancellationis formed in the channel estimation means 8. Each of the blocks 1-8 isalso provided with a sufficient amount of memory.

FIG. 2 shows an interference cancellation primitive 10 by which theinterference cancellation means 5 is implemented particularly when theBPSK modulation is used. The interference cancellation primitive 10comprises a selection means 11 by which two consecutive bits b_(n) andb_(n±1) from the five bit window of the interfering signal are chosen bya bit index window bit_window_index. The window comprises five bits asthe cross correlations R_(I,I′,i−i′) (k, k′) obtain the value 0 when|i−i′|>2. These two bits are typically in such a phase that theyinterfere with one bit of the desired signal, as illustrated in FIG. 5.Both of the selected bits are multiplied in order to form a weightedestimation result in a multiplier 12 by a cross correlation 16 and 17between the spreading codes of a desired signal and an interferingsignal, the cross correlation functioning as a weight value in such amanner that the cross correlation is the greater the more theinterfering signal interferes with the desired signal. The crosscorrelations corresponding to both bits b_(n) and b_(n±1) are r_(n) andr_(n±1) 16 and 17. The interference magnitudes are summed in a summer 13whereby a total interference magnitude is formed. The total interferencesize is multiplied in the multiplier 12 by a complex channel estimateR_chan_est and J_chan_est 18 and 19. Then a complex interfering estimatesignal 14 is generated comprising a real interfering estimate signal Rand an imaginary interfering estimate signal I.

It is assumed in the solution in FIG. 3 that a complex channel estimateR_chan_est and J_chan_est 18 and 19 remains stable during several bits.Then the channel estimate R_chan_est and J_chan_est 18 and 19 and thecross correlation r_(n±1) and r_(n) 16 and 17 of the desired signal andthe interfering signal also remaining stable during several bits, canpreliminary be multiplied as R_chan_est*r_(n), R_chan_est*r_(n±1),J_chan_est*r_(n) and J_chan_est*r_(n±1) and store the results in memory.Then both consecutive interfering bits are multiplied by the product ofthe channel estimate and the cross correlation in a complex way and acomplex interfering estimate signal is generated by summing theinterference effects of both bits. At this time the multipliers 12 arepreferably inverters as multiplication by a bit is performed by changingthe character of the multiplicand, when the bit is mapped into the group[−1,+1].

FIG. 4 shows the solution of the invention when the QPSK modulation isused. Then the interference cancellation primitive comprises a selectionmeans 11 to select two symbols from a five symbol window using a symbolwindow index symbol_window_index. After this the bits included in thesymbols are preferably dealt with in the same way as in the case of theBPSK modulation. Eight cross correlation values r_(Qn, Q), r_(Qn, I),r_(In, Q), r_(In, I), r_(Qn±1, Q), r_(On±1, I), r_(In±1, Q) andr_(In±1, I), 16 and 17 are thus needed as one QPSK symbol comprises twobits, of which a Q and an I cross correlation result is needed for bothbits, and where the I and Q symbols stand for the I/Q symbols of theQPSK modulation. In other words, one symbol of the desired signal iscorrelated with two interfering signal symbols, as in the case of theBPSK modulation, but both bits of the desired signal are correlated withfour interfering signal bits. Cross correlation r_(Qn, Q) stands for thecross correlation n of the Q symbol of the interfering signal with the Qsymbol of the desired signal, r_(Qn, I) stands for the cross correlationn of the Q symbol of the interfering signal with the I symbol of thedesired signal, r_(In, Q) stands for the cross correlation n of the Isymbol of the interfering signal with the Q symbol of the desiredsignal, r_(In, I) stands for the cross correlation n of the I symbol ofthe interfering signal with the I symbol of the desired signal,r_(In±1, I) stands for the cross correlation n±1 of the I symbol of theinterfering signal with the I symbol of the desired signal, r_(Qn±1, q)stands for the cross correlation n±1 of the Q symbol of the interferingsignal with the Q symbol of the desired signal, r_(Qn±1, I) stands forthe cross correlation n±1 of the signal Q of the interfering symbol withthe I symbol of the desired signal, r_(In±1, Q) stands for the crosscorrelation n±l1 of the I symbol of the interfering signal with the Qsymbol of the desired signal and r_(In±1, I) stands for the crosscorrelation n±1 of the I symbol of the interfering signal with the Isymbol of the desired signal. The channel estimate is complex comprisingan imaginary part J_chan_est 18 and a real part R_chan_est 19. Thissolution can also be simplified in the same way as in the case of theBPSK modulation in FIG. 3, when the channel estimate is slowly changingin comparison with the duration of the symbols.

FIG. 5 shows a simplified situation on how the interfering signalsymbols interfere with the desired signal symbol. The symbol is a bit ora combination of bits. The interfering signal comprises consecutivesymbols (b_(n) and b_(n±1)) 20 and 21. The desired signal comprises asymbol 22 which is divided into parts 22 a and 22 b as they appear inthe phase shift with the interfering signal symbols 20 and 21. Thefigure shows that in interfering cancellation the symbols 20 and 21 oftwo interfering signals are to be preferably removed from the desiredsignal symbol 22, and 22 a and 22 b in so far as they interfere with thedesired signal symbol 22.

FIG. 6 shows a preferred embodiment of the invention. In this figure thereal part (and the imaginary part I of the QPSK modulation are not shownseparately but the signals are to be assumed to be complex. Theinterference cancellation means 5 of the solution comprises a set ofinterference cancellation units 31-33 comprising interferencecancellation primitives 10. In addition, the receiver of the inventioncomprises a routing means 34 and a combining means 6. The combiningmeans 6 comprises a set of user-specific path combining means 13. Inthis adaptive solution there is preferably a predetermined number ofinterference cancellation units 31-33 and interference cancellationprimitives 10. Each interference cancellation primitive 10 generates aninterfering signal estimate and each interference cancellation unit31-33 generates, in turn, a cleaned signal 35 of one path I.Interference cancellation can preferably be performed more than once,when new interfering signal estimates are generated using interferencecleaned signals and said new interfering signal estimates 14 are removedfrom a narrowband signal 15. Then after the first interferencecancellation, bit decisions that are more reliable than the bitdecisions originally made, are made from the received signal. Usingthese more precise bit decisions a better channel estimate can, in turn,be formed and together with cross correlation a better interferingestimate signal can be generated. When the interfering estimate signalsthus generated are reduced from the received total increasingly cleaneruser signals are obtained. A multiple interference cancellation ispreferably performed three times at the most. The routing means 34directs the interference cleaned signal 35 of each path adaptively tothe user-specific combining means 6, in which paths L of each subscriberk are combined into one signal 38-40. This solution comprises preferablyK*L interference cancellation units, in which K indicates the number ofusers and L is the average number of paths of each user. This solutionworks also with a smaller number of interference cancellation units,whereby example an L interference cancellation unit 31-33 is needed. Acontrol unit of the receiver preferably directs a new interfering pathto a free interference cancellation unit 31-33. Similarly theinterference cancellation unit 31-33 is released when the interferingsignal weakens. In this solution different users can have a varyingnumber of paths, as the signals of the different paths can adaptively bedirected to a free interference cancellation unit.

The solution of the invention can also be implemented in parallel asshown in FIG. 7. In this figure the real part Q and the imaginary part Iof the QPSK modulation are not shown separately but the signals are tobe assumed to be complex. Means 41 and 42 generate a user-specificinterference-free signal; the number of the means needed must equal thenumber of the users, i.e., K. The means 41 and 42 comprise interferencecancellation units 31-33 and the combining means 6. In this solutionthere are a lot of interference cancellation primitives 10: (K*L) *(K*L−1) where K is the number of users and L is the number of paths ofeach user. Each user k has preferably an L number of interferencecancellation units 31-33 which provide an interference cleaned signal 35from each path I. The interference cleaned signals of the paths L ofeach user k are combined with the combining means 6. This solution is,as the adaptive solution in FIG. 6, a very fast interferencecancellation method and the interference-free signals of all users areready at the same time.

In FIG. 8 interference cancellation is performed in serial mode. In thissolution the interfering estimate signals of each interferencecancellation primitive 10 are summed consecutively in a summer 13 andsubtracted from the narrowband composite signal in the last summer 13.This FIG. 8 shows the complex operation of the I/Q modulation separatelyfor the real part 12 and the imaginary part I. Each interferencecancellation primitive 10 generates an interfering signal estimate ofone path I. In this solution the interference cleaned signals of eachuser k and each path I are generated consecutively. In order to be ableto clean the desired paths of all users each interference cancellationprimitive 10 should be used K*L times. This solution preferably enablesthe use of already interference removed bit decisions in theinterference cancellation of the next path.

FIG. 9 shows a radio system to which the solution of the invention canpreferably be applied. The radio system comprises a base stationcontroller 50, base stations 51, and subscriber terminals 52. The basestation controller 50 communicates through a digital link 55 with theother parts of the system, for example, with a mobile services switchingcenter (not shown in FIG. 9). The base stations 51 communicate through adigital link 54 with the base station controller 50. The subscriberterminals 52, which are preferably mobile phones, and the base stations51 comprise a transmitter and a receiver to transmit and receivewideband signals 53 propagating on a plurality of paths and which aredesired signals that also cause interference to one another. From areceiver point of view, the user is the transmitter, whereby the signalsof several users are the signals of several transmitters. Thus, thecomposite signal 9 received by the receiver comprises a set of widebandsignals 53 transmitted by individual transmitters. The radio system ofthe invention preferably uses DS-CDMA signalling in the signals 53, andone speed can be used as a data transmission speed or the datatransmission speed can be varied. The solution of the invention isparticularly suited to be used at the base station of the radio system.

The solutions of the invention can be implemented particularly regardingdigital signal processing, for example, by ASIC or VLSI circuits.Functions to be performed are preferably implemented as software.

Even though the invention has been described with reference to theexample of the accompanying drawings, it is obvious that the inventionis not restricted to it but can be modified in various ways within thescope of the inventive idea disclosed in the attached claims.

I claim:
 1. A receiving method used in a CDMA radio system including, asa transmitter and a receiver, at least one subscriber terminal and abase station, which receive a composite signal that has been multipliedby a spreading code into wideband, the composite signal arriving at thereceiver over several paths from a plurality of transmitters, thecomposite signal then representing both interfering signals and desiredsignals, the method comprising: converting the wideband composite signalinto a narrowband composite signal at the receiver; and subtracting anarrowband interfering signal estimate of one path of at least onetransmitter from the narrowband composite signal propagated over severalpaths to generate an interference removed signal.
 2. The method of claim1, further comprising: multiplying a received interfering signal of atleast one path of at least one transmitter by a cross correlation ofspreading codes used in the interfering signal and the desired signal,whereby an estimate result weighted by the interference size is formed;and multiplying the weighted estimate result by a channel estimate,whereby the narrowband interfering signal estimate is generated.
 3. Themethod of claim 1, wherein, when the composite signal includes symbolsand when the desired signal and the interfering signal are in phaseshift regarding each other, the interfering signal estimate representingmore than one interfering signal symbol is subtracted from thenarrowband composite signal to the extent that the interfering signalsymbols interfere with one symbol of the desired signal, is subtractedfrom the narrowband composite signal.
 4. The method of claim 1, wherein,when a total number of paths included in the method is predetermined andwhen strengths of the signals of different paths forming the receivedcomposite signal propagated over several paths is measured, the methodfurther comprises subtracting a predetermined number of interferingsignal estimates of the strongest paths from the narrowband compositesignal.
 5. The method of claim 1, wherein, when the signals ofparticular paths represent the signal of a particular transmitter, theinterference removed signal of each path is generated by subtracting theinterfering signal estimate separately from the narrowband compositesignal and combining the interference removed signals of different pathsthus generated into transmitter-specific signals.
 6. The method of claim1, further comprising subtracting the narrowband interfering signalestimate from the narrowband composite signal more than once in such amanner that new interfering signal estimates are generated using alreadyinterference removed signals and the new interfering signal estimatesare removed from the narrowband composite signal.
 7. The method of claim1, wherein, when the composite signal includes symbols, the interferingsignal estimate of one path is generated from the received narrowbandsignal one symbol at a time and is subtracted from the narrowbandcomposite signal one symbol at a time.
 8. The method of claim 1, whereinthe interference removed signal of one path is generated by subtractingthe narrowband interfering signal estimates of interfering paths fromthe narrowband composite signal.
 9. The method of claim 8, furthercomprising generating interference removed signals of several pathssuccessively in a serial mode.
 10. The method of claim 8, furthercomprising generating interference removed signals of several paths inparallel.
 11. A receiver arranged to be used in a CDMA radio systemincluding at least one subscriber terminal and a base station which arearranged to receive a wideband composite signal including signalspropagated over several paths from a plurality of transmitters, andwhich wideband composite signal includes both interfering signals anddesired signals, and which wideband composite signal the receiver isarranged to convert into a narrowband composite signal, the receivercomprising: an interference cancellation means including a plurality ofinterference cancellation primitives which are arranged to generate atleast one narrowband interfering estimate signal describinginterference, the interference cancellation means being arranged tosubtract a narrowband interfering estimate signal from the narrowbandcomposite signal.
 12. The receiver of claim 11, wherein, when thewideband composite signal includes symbols, one interferencecancellation primitive is arranged to generate the narrowbandinterfering signal estimate of one path of one receiver in such a mannerthat the interference cancellation primitive is arranged: to choose twoconsecutive symbols from a five symbol window of an interfering signal;and to generate the interfering estimate signal as a product of theconsecutive symbols of the interfering signal based on a channelestimate of the interfering signal, based on a cross correlation of theinterfering signal and the desired signal and based on the twoconsecutive symbols of the interfering signal so that a common effect ofthe two symbols on one symbol of the desired signal is taken intoaccount in the product by summing the product of both of the two symbolsand the cross correlation corresponding to the two symbols or a commonproduct of both of the two symbols, the cross correlation of theinterfering signal and the desired signal and the channel estimate ofthe interfering signal.
 13. The receiver of claim 11, wherein thereceiver is arranged to measure strengths of interfering signals, thereis a predetermined number of interference cancellation primitives, thereceiver is arranged to choose a predetermined number of the strongestinterfering signal estimates to be removed from the narrowband compositesignal; and the receiver is arranged to combine interference canceledsignals of interference cancelled paths transmitter-specifically. 14.The receiver of claim 11, further comprising an interferencecancellation means arranged to subtract interfering signal estimatesfrom the narrowband composite signal at least twice successively togenerate new interfering signal estimates using interference cleanedsignals, the new interfering signal estimates being removed from thenarrowband composite signal.
 15. The receiver of claim 11, wherein theinterference cancellation means is arranged to generate an interferencecancelled signal of one path by subtracting narrowband interferingsignal estimates of interfering paths from the narrowband compositesignal.
 16. The receiver of claim 15, wherein the interferencecancellation means is arranged to generate interference cancelledsignals successively in a serial mode.
 17. The receiver of claim 15,wherein the interference cancellation means is arranged to generateinterference cancelled signals substantially in parallel.
 18. Areceiving method used in a radio system including, as a transmitter anda receiver, at least one subscriber terminal and a base station, whichreceive a wideband composite signal, the wideband composite signalarriving at the receiver over several paths from a plurality oftransmitters, the composite signal then representing both interferingsignals and desired signals, the method comprising: converting thewideband composite signal into a narrowband composite signal at thereceiver; and subtracting a narrowband interfering signal estimate ofone path of at least one transmitter from the narrowband compositesignal propagated over several paths to generate an interference removedsignal.
 19. A receiver arranged to be used in a radio system includingat least one subscriber terminal and a base station which are arrangedto receive a wideband composite signal including signals propagated overseveral paths from a plurality of transmitters, and which widebandcomposite signal includes both interfering signals and desired signals,and which wideband composite signal the receiver is arranged to convertinto a narrowband composite signal, the receiver comprising: aninterference cancellation means including a plurality of interferencecancellation primitives which are arranged to generate at least onenarrowband interfering estimate signal describing interference, theinterference cancellation means being arranged to subtract a narrowbandinterfering estimate signal from the narrowband composite signal.